This invention relates to furnace systems and more particularly to furnace systems, which employ an overfire air (OFA) process to reduce harmful by-products, such as CO, NOx and unburned carbon products.
The complete combustion of fossil fuels or other types of organic and chemical fuels in a furnace requires a fixed and known quantity of combustion air. The relationship between air and fuel is known as stoichiometric combustion conditions. Because the supply of stoichiometric air to the combustion and subsequent consumption of fuel is theoretical, a furnace of infinite size would be required to achieve complete combustion. In existing furnaces, more air is supplied than is theoretically required. This additional quantity is referred to as excess air.
In the absence of such excess air, significant quantities of by-products are produced due to incomplete combustion. Such by-products include hydrocarbons (HC) and carbon monoxide (CO). Although the use of excess air helps to eliminate undesirable HC and CO by-products, during combustion at the burners the excess O2 combines with nitrogen (N) released from fuel particles to form nitrous oxides (NOx), harmful pollutants that permeate into the atmosphere upon exit from the furnace.
The overfire air (OFA) process was developed in the 1950s to reduce NOx output. The OFA process is an air staging process that regulates the supply of air necessary to complete the combustion process. Typically, the OFA process occurs in two stages.
The first stage requires removal of a portion of the combustion air from the combustion zone, where the burners are located. Removing a portion of the combustion air allows for the combustion process to begin under fuel-rich conditions. Such conditions result in a significant reduction and prevention of the formation of NOx, but simultaneously cause the formation of high levels of carbon monoxide (CO) and unburned carbon products (UBC).
The second stage of the OFA process remedies this shortcoming. In this stage, the removed air is injected through OFA ports located above the combustion zone, or in the CO burnout zone. The injection of the removed air in the CO burnout zone provides the stoichiometric amount of air necessary for complete combustion to occur. Ultimately, CO oxidizes to form CO2.
Use of the OFA process therefore provides the balance necessary to reduce the formation of harmful NOx and CO.
Combustion efficiency is affected by various factors including the time that the fuel source is exposed to a flame, the temperature and turbulence (i.e., mixing between the air and fuel particles). Various prior art furnace systems exist, which include OFA ports and other features affecting the amount of time, temperature and mixing necessary for effective combustion. These variables include the number of OFA ports, the location of such ports relative to the combustion zone, the design of the OFA ports (e.g., single stage and dual stage port design) and various mixing methods.
To address the problem of insufficient mixing, “two stage” or “dual throat” OFA port designs have been implemented. Such designs are intended to create a “near zone” flow field that causes rapid mixing between the OFA flow and the furnace gases close to the injection wall. This is generally accomplished by causing the air in the outer throat or stage to swirl. Further, high velocity axial air flow from the inner stage or throat permits the OFA to penetrate sufficiently far into the furnace, thereby achieving greater mixing in the interior of the furnace. Prior art two stage OFA ports are subject to various problems. One of the defects of the swirling outer flow is that rotational flow results in up-flow along one side of the port and down-flow on the other side. Because the mixing is not symmetrical about the vertical centerline of the port, unmixed furnace gases are permitted to pass by the port yielding undesirable amounts of CO and other by-products of incomplete combustion, which flow out of the furnace.